1. Field of the Invention
The present invention relates to a wire for an Nb.sub.3 X superconducting wire, and more particularly, it relates to a wire for an Nb.sub.3 X superconducting wire which is employed as a high magnetic field superconducting material such as a superconducting magnet for a nuclear fusion reactor or the like.
2. Description of the Background Art
In general, an Nb.sub.3 X superconducting material such as Nb.sub.3 Al, Nb.sub.3 Sn or Nb.sub.3 Ge is expected as a superconducting material since the same is applicable to a high magnetic field use, which cannot be satisfied by an alloy superconducting material such as NbTi. In particular, an Nb.sub.3 Al superconducting material is expected as a useful superconducting material for a nuclear fusion reactor which receives high magnetic force in a high magnetic field, or a superconducting material for power storage, due to a high critical current and excellent stress-strain effect in a high magnetic field.
In general, it is difficult to plastically work such an Nb.sub.3 X superconducting material, which is extremely hard and brittle as compared with an alloy superconducting material such as NbTi. Thus, various studies have generally been made on a method of preparing such an Nb.sub.3 X superconducting material.
FIG. 10 is an Nb-Al binary alloy phase diagram.
Referring to FIG. 10, Nb.sub.3 Al stably exists under a high temperature of at least 1600.degree. C. with a stoichiometric composition ratio of Nb to Al of 3:1. Under a low temperature of less than 1600.degree. C., however, the Nb.sub.3 Al is formed with mixture of Nb.sub.2 Al, which is a non-superconducting material. Therefore, an Nb.sub.3 Al compound superconducting material which is prepared at a low temperature of less than 1600.degree. C. has a low critical temperature, a low critical magnetic field (Hc) and a low critical current density (Jc) in general. Thus, no method of preparing an Nb.sub.3 Al superconducting material has been put into practice although there has been studied a quenching method of cooling the material from a high temperature of at least 1600.degree. C. in a short time or the like.
However, it has recently been recognized that excellent Nb.sub.3 Al is formed in a portion having a short diffusion length of Al atoms in an Nb layer and/or a portion having a short diffusion length of Nb atoms in an Al layer, even if the temperature is not more than 1000.degree. C. To this end, there have been developed production methods such as powder metallurgy (PM), composite working such as a tube method and clad chip extrusion, and a jelly roll method. Every one of these methods is adapted to mix pure Nb or an Nb alloy with pure Al or an Al alloy in an extremely fine state, and can increase the aforementioned portion having a short diffusion length. According to such a method, therefore, it is possible to obtain a high-performance Nb.sub.3 Al superconducting material having a stoichiometric composition ratio of Nb to Al which is close to 3:1, with a high critical temperature, a high critical magnetic field (Hc) and a high critical current density (Jc).
Among the aforementioned preparation methods, the jelly roll method can advantageously prepare a superconducting wire having a multifilamentary structure, compose a stabilizing material, and easily prepare a long material. Thus, this is the most practicable method of preparing an Nb.sub.3 Al superconducting wire.
The jelly roll method is now described with reference to FIG. 11.
FIG. 11 is a process drawing showing a method of preparing an Nb.sub.3 Al superconducting multifilamentary wire in accordance with the jelly roll method.
Referring to FIG. 11, a high-purity Nb sheet and a high-purity Al sheet are first prepared by melting and rolling. Then, the Nb sheet and the Al sheet are superposed with each other and wound on an oxygen free copper rod, to prepare a core (jelly roll). Then, this wire is inserted in an oxygen free copper pipe and drawn into a hexagonal shape in section, and thereafter cut into a desired length to prepare a hexagonal segment 150. Then, a plurality of such hexagonal segments 150 are filled in a copper pipe to prepare a billet, which in turn is sealed up in a vacuum by electron beam welding, and subjected to extrusion. Then, the as-formed multifilamentary wire is drawn and twisted, shaped, stranded and insulated at desire, coiled and thereafter heat treated to form a superconducting phase (Al5 structure of Nb.sub.3 Al). The aforementioned jelly roll method is described in more detail in Sumitomo Denki, No. 139, September 1991, pp. 93-100.
In order to form Nb.sub.3 X having excellent superconductivity by such a jelly roll method, it is necessary to maximize the amount of working such as drawing for reducing the sectional area and reduce first and second sheets in thickness, for increasing the amount of the aforementioned portion having a short diffusion length.
Thus, an Nb.sub.3 X superconducting wire prepared by the jelly roll method comprises a wire which is formed by superposing and winding up a first sheet consisting of Nb or an Nb alloy and a second sheet consisting of metal atoms X, which react with Nb for forming a superconductive compound, or an X alloy, and a stabilizing material layer which is so provided as to enclose the wire, and the first and second sheets are worked to be extremely small in thickness respectively. The stabilizing material is prepared from copper or a copper alloy, for example.
When such an Nb.sub.3 X superconducting wire prepared by the jelly roll method is heat treated to form Nb.sub.3 X, however, the metal atoms X are thermally diffused as impurities into the copper or the copper alloy which is employed as a stabilizing material and react with the copper atoms to form a compound. Thus, a residual resistance ratio of the Nb.sub.3 X superconducting wire is disadvantageously deteriorated.
In order to solve the aforementioned problem, there has been proposed a wire for an Nb.sub.3 X superconducting wire having a diffusion barrier layer which is provided between an outer surface of a wire prepared by superposing and winding up first and second sheets and an inner surface of a stabilizing material layer for preventing metal atoms X from diffusion into the stabilizing material. In such a wire for a superconducting wire, the diffusion barrier layer is generally formed by the first sheet which consists of pure Nb or an Nb alloy. Namely, the first sheet is prepared from a sheet which is longer than the second sheet along the direction of winding so that the same is superposed and wound up with the second sheet to form a laminate portion, and the remaining portion of the first sheet is further wound on the outer surface of the laminate portion, to form the diffusion barrier layer.
When a core which is formed by superposing and winding up the first and second sheets is worked to be reduced in sectional area, however, the diffusion barrier layer may also be reduced in thickness such that the same cannot sufficiently serve its function after the degressive working. The thickness of the diffusion barrier layer depends on factors such as the heat treatment situation and the diffusion coefficient of the material therefor, while it has been regarded preferable that its thickness is at least 1 .mu.m after the degressive working. When the diffusion barrier layer is increased in thickness, however, the sectional area of the Nb.sub.3 X superconducting wire per sectional area of a non-copper part is relatively reduced to disadvantageously reduce the critical current density (Jc).